In the production of starch from wheat grain, it is common practice, firstly, to mill the grain to separate the bran and fiber, and then to treat the wheat flour obtained to isolate starch and gluten (protein) fractions. A number of methods have been proposed for this latter separation but two in particular may be singled out for mention. The first involves mixing the flour with water to form either a dough or batter and using water to extract the gluten by a physical separation often involving centrifugal action. This process suffers, however, from the disadvantage that a clean-cut separation of starch and gluten is not obtained even when a sophisticated separation system is employed, e.g., a multistage separation system using a bank of hydrocyclones. Generally, some of the starch obtained is of inferior quality and is unsuitable for use as a feedstock for hydrolysis to oligo- and monosaccharides, such as glucose.
A second method which has been proposed for the separation of starch and gluten is a chemical treatment in which the gluten is dispersed or dissolved in an alkali solution from which it is subsequently liberated by acidification. Typical alkalis which may be used include sodium hydroxide, potassium hydroxide, calcium hydroxide and barium hydroxide; sodium hydroxide being preferred. In such a process, the wheat flour is typically slurried with sodium hydroxide and the slurry subjected to a centrifugal treatment to separate the solid starch from the protein solution. The starch obtained in this way contains all the alkali-insoluble solids of the flour, e.g., cellulosic cell-wall fragments and bran. In addition, there is some gelatinized starch present arising from particles damaged by milling. This starch may subsequently be treated, e.g., by centrifugation or by "tabling" (a form of decantation) to produce a prime quality starch and inferior quality starch. The gluten from the process is, however, of inferior quality compared with that obtained by the water-washing process and dual quality starch is obtained.
It has also been proposed, in U.S. Pat. No. 3,790,553, to use a water extraction of the gluten in conjunction with alkali treatment of the starch. In the process described in the U.S. patent, the whole wheat kernel is crushed and slurried with water which dissolves the low molecular weight proteins. The residue from the first step, containing the starch, is then slurried with a mild base, preferably ammonium hydroxide, and centrifuged to obtain phase separation. The supernatant liquid from the separation contains the high molecular weight protein, while the residue contains the starch and any other residual alkali insoluble products. Strongly basic alkalis are not recommended for use in the process of the U.S. patent since they are said to affect adversely the quality of the protein obtained. The process described in U.S. Pat. No. 3,790,553 is primarily directed at obtaining high quality protein from whole wheat kernel and is less concerned with the quality of the starch.
Another modification of the aqueous starch/gluten separation process uses an enzymatic treatment to recover all of the starch. This process is described in the Canadian Journal of Technology, Vol. 33, pp. 33 to 40, in an article entitled "Separation of Starch and Gluten, VII. The Application of Bacterial Pentosanases to the Recovery of Starch from Wheat Flours". In the process described in this article, wheat flour is extracted with water to yield gluten and a starch milk. The starch milk is then centrifuged to give product in the centrifuge consisting of a hard starch and an inferior quality "squeegee starch" on top of the hard starch. The latter is scraped off the hard starch and consists of strongly hydrated pentosan masses in which starch granules, nitrogenous materials and fiber are embedded. The process described uses a bacterial pentosanase to hydrolyze the pentosans under carefully-controlled conditions, so releasing the embedded starch granules. The process described in the Canadian journal was only investigated on a laboratory scale, and problems, such as enzyme deactivation, enzyme color, and possible enzyme affects on the starch and gluten, were not addressed.
We have now devised a process for the separation of starch and gluten which produces gluten of high quality (so-called "vital gluten") and higher in quality than that produced by the alkali treatment of wheat flour described above. The process also produces starch of high quality, substantially all of which may be used as a feedstock for subsequent hydrolysis processes. In addition, the process is simple to operate on a commercial scale and avoids the uncertainties involved in using a pentosanase.
The process of the invention is a modification of the water extraction process described earlier in this specification. As has been mentioned in connection with that method, it is not possible on a commercial scale to obtain a clean-cut separation of the starch and gluten, and depending upon the separation process adopted and its method of operation, either the gluten fraction contains some starch or the starch fraction contains some gluten. In both cases, a subsequent separation step is required to obtain pure starch or pure gluten, and, in both cases, the by-product is a poor quality starch which typically contains 75-95% by weight starch together with some gluten but also other impurities, such as pentosan and hemicellulose. The process described in the Canadian Journal of Technology referred to above obtains the high quality starch and low quality or "squeegee starch" together in the centrifuge, and the latter is then physically separated, e.g., by a scraper. We have found that in a process in which the starch and gluten are separated in a hydrocyclone battery, some starch is carried over with the gluten from which it may be separated in a subsequent process step, e.g., by using a screen. The residual starch which is separated from the gluten in this way is similar in nature to the "squeegee starch". For convenience, the main starch product which is of good quality may be referred to as `A` starch, while the "squeegee" or inferior quality starch may be called `B` starch.